The present invention relates to a cushion valve arrangement for cushioning pressure surges in a hydraulic system.
Relief valves are commonly used in hydraulic systems where a fluid actuated device, such as a motor or piston-and-cylinder moves or controls or is subjected to a heavy load and may be started and stopped suddenly, thus developing a pressure surge in the system. More specifically, in the case of fluid-linked power steering systems, a sudden opening of the control valve may cause severe shocks to the system components. It is well known that a relief valve, when subjected to a sudden surge of pressure, will not react quickly enough, so that for a short interval of time the fluid pressure will rise above the relief valve setting. The resulting shock wave is often of sufficient magnitude to damage various components of the hydraulic circuit.
Typical relief valves which open when the system pressure rises above the pressure setting remain open as long as the system pressure is higher than the setting. While such a relief function is important, and may be an additional feature of the arrangement of the present invention, the invention is a true cushion valve. As used herein, the term "cushion" will be understood to mean reducing the pressure rise rate to minimize the effect of pressure surges at pressures below the relief valve setting. Although the cushion valve arrangement of the present invention may be used in any type of hydraulic system subjected to transient pressure shocks or pressure surges resulting from the starting, stopping or reversal of high inertia loads, it is especially adapted for use in fluid-linked power steering systems, and will be described in connection therewith.
Examples of prior art arrangements referred to incorrectly as "cushion" valves include U.S. Pat. Nos. 3,367,354 and 3,414,006. The former is illustrated in a steering system, but it is actually a surge relief valve, disposed upstream from the steering control valve, such that the entire system flow goes through the surge relief valve. In addition, the surge relief valve disclosed in the U.S. Pat. No. 3,367,354 is primarily a relief valve which opens in response to a relatively high pressure signal and thus, does not prevent pressure surges below the relief setting from damaging system components when the steering control valve is suddenly opened or suddenly closed. The surge relief valves taught in the cited prior art both have the additional disadvantage of being one-directional, i.e., they would not be operative if connected in parallel between the two conduits communicating between a steering control valve and a power steering cylinder. Also, valves of this type are not effective to prevent a jerky pressure rise below the relief setting, thus permitting jerky steering action.
Some of these disadvantages are overcome by the relief valve arrangement shown in U.S. Pat. No. 3,330,298, assigned to the assignee of the present invention. The valve arrangement of the cited patent still has the disadvantage of the entire system flow passing through the valve. Furthermore, because the valving arrangement utilized to relieve pressure surges is duplicated for each direction of operation, the overall valve has an excessive number of moving parts, valve members, valve seats, springs, etc..